A typical UPS is one in which a primary energy source (normally a commercial AC line) and a reserve energy source (normally a battery voltage driven inverter) are both coupled to supply energy input to an energy coupling apparatus such as a transformer with two independent input windings. Each input is functional to supply power to a single output which in the case of a transformer is a single output winding. In other arrangements the two sources of energy may be joined in series so that the primary source of energy charges the reserve source. Frequently one or more direct bypass current lines are provided to directly couple thge primary energy source to the output to supply temporary surge power to the output.
When power is being supplied from the reserve energy source, the UPS is disconnected from the commercial AC line by semiconductor electronic isolation switches located in both the input and bypass lines. Electronic isolation switches are used to avoid the transients associated with the switching of mechanical switches. Should these electronic isolation switches fail however in a conducting mode, power supplied by the reserve energy source may be fed back into the primary line source either directly through the energy coupling apparatus from the inverter driven by the reserve energy source or from the UPS output via the bypass line depending upon the power transfer topology used. There generally is no indication of this failure until a current path is accidentally completed across the AC line permitting such current flow. This is a potential safety hazard which is often not apparent until such a current path is completed.